1. Field of the Invention
The present invention relates to a process for the delignification and bleaching of a lignocellulose material by an aqueous solution of an oxidizing agent and of an oxidation-reduction catalyst.
The invention therefore relates to the technical field of wood and of paper pulp and also to that of natural or synthetic oxidation-reduction catalysts.
2. Background Art
Many studies relate to processes for the degradation of wood lignin or paper pulp.
As restated by A. Paszcynski et al., Applied and Environmental Microbiology, 1988, 62-68, many of these studies are based on the study of natural wood degradation phenomena.
Thus a specific strain of basidiomycetes, known under the name of "Phanerochaete chryosporium" and leading to white rot of wood, was particularly studied as regards the biochemical mechanisms which are involved in delignification.
Two types of extracellular enzymes containing porphyrin groups are involved in the degradation of wood.
These enzymes have in common the ability to decompose peroxides, which ability is characteristic of peroxidases; their catalytic cycle comprises the oxidation of the iron porphyrin by the peroxide and then the return to the initial state by virtue of electrons withdrawn from the molecules present in the medium.
Ligninases capture an electron from the aromatic rings of lignin and thus form radical cation species which progress non-enzymatically and which lead to depolymerization of lignin.
Manganese peroxidases oxidize Mn.sup.2+ cations to Mn.sup.3+ cations. The Mn.sup.3+ cations diffuse into the structures of wood and capture an electron to restore Mn.sup.2+.
The above writers treated wood shavings or paper pulp with a number of natural or synthetic iron porphyrins in the presence of an oxidizing agent chosen from hydrogen peroxide or tert-butyl hydroperoxide (TBH), in an aqueous medium.
The experiments were carried out with a wood/water ratio by mass of 0.2% and a pulp/water ratio by mass of 0.1% at approximately 100.degree. C. (reflux temperature of water) for 24 to 48 h.
The chosen kraft pulp, treated with an iron hemin/TBH, sees its kappa number decrease from 36 to 2 with removal of 100% of the lignin and only 10% of the cellulose.
On the other hand, wood shavings treated under identical temperature and time conditions with hydrogen peroxide alone or with the H.sub.2 O.sub.2 /hemin combination lead to the same results, namely a degradation of the lignin and cellulose which is non-selective and of the same order of magnitude. Heroin does not seem to play any specific role in the presence of H.sub.2 O.sub.2.
On the other hand, in the presence of TBH, by refluxing for 48 h, 38% of the lignin is removed with concomitant removal of 9.5% of the cellulose.
In concluding their studies, these writers recognize that their process cannot be applied industrially.
For their part, P. S. Skerker et al., Biotechnology in Pulp and Paper Manufacture, Chap. 18, 203-210, Butterworth, Heinemann (1990), have studied the biomimetic bleaching of kraft pulps using synthetic porphyrins in the presence of TBH. The porphyrin which is soluble in water and which is resistant to oxidation is mesotetra -(2,6-dichloro-3-sulphonatophenyl)-.beta.-octachloroporphinatoiron(III).
It should be noted that this porphyrin comprises 16 chlorine atoms substituting the phenyl and pyrrole rings and 4 sulphonato groups and that its synthesis is very expensive. Replacement of iron by manganese in this porphyrin structure does not substantially change the results.
Having a pulp with a consistency of 2.5%, a delignification of better than 45% is obtained in 15 min at 80.degree. C. in water with the above Fe(III) porphyrin, the kappa number decreasing from 23.5 to 18.7. However, these conditions lead to a decrease in the viscosity of the pulp by a factor of approximately 2, which is a sign of severe depolymerization of the cellulose.
In their conclusion, these writers themselves recognize that their process cannot be operated industrially.
Forrester et al., Biochemical and Biophysical Research Communications, Vol. 157, no. 3, 992-999, 1988, were concerned with the second type of enyzmes which are involved in the delignification caused by Phanerochaete chrysosporium. These writers have shown that a simpler biomimetic system, consisting of the Mn.sup.3+ cation complexed with pyrophosphate and in the presence of reduced glutathione, oxidized veratryl alcohol to the corresponding aldehyde and also caused delignification of wood fibres.
With veratryl alcohol, if hydrogen peroxide is added to the above reaction medium, approximately 3 times less veratraldehyde is obtained.
Apart from the technical field of wood and paper pulp; other natural metal complexes involved in biological oxidations have formed the subject of publications.
Saver, Acc. Chem. Res., 13, 249 (1980) and the references cited, restates that complexes containing two manganese atoms can be involved in biological oxidations.
Hodgson et al., Inorganica Chemical Acta, 141 (1988), 167-168, reviews synthetic complexes containing two manganese atoms and describes, in particular, some in which the metal is in high oxidation states.
The 2,2'-bipyridine (bpy) complex of formula [(bpy).sub.2 MnO].sub.2.sup.3+ and its 1,10-phenanthroline (phen) analogue are known. These complexes have been isolated in the form of stable solids in the Mn(III)/Mn(IV) oxidation state and have been studied in solution in their Mn(IV)/Mn(IV) complete oxidation state.
The phen complex has been isolated as a solid in the Mn(IV)/Mn(IV) state and its structure characterized by M. Stebler et al., Inorg. Chem., 25, 4743 (1986).
D. J. Hodgson et al., in the above publication, report the synthesis of a new complex (I): EQU [TPA)Mn(III)O.sub.2 Mn(IV)(TPA)].sup.3+ (I)
in which TPA is a tetradentate ligand consisting of tris(pyridyl-2-methyl)amine.
The complex (I) is obtained by mixing TPA and MnSO.sub.4 .multidot.H.sub.2 O in water and then adding hydrogen peroxide. By addition of sodium dithionate, the salt [(TpA)MnO].sub.2 .multidot.(S.sub.2 O.sub.6).sub.3/2 .multidot.7H.sub.2 O crystallizes and its structure is determined by X-ray crystallography.
Uehara et al., Chemistry Letters, 1988, 477-480, have isolated the complexes: EQU [(TPA)Mn(III)O.sub.2 Mn(IV)(TPA)].sup.3+ (ClO.sub.4.sup.-).sub.3- H.sub.2 O (II) EQU [(TPA)(Mn)(IV)O.sub.2 (Mn)(IV)(TPA)].sup.4+ (ClO.sub.4.sup.-).sub.4- CH.sub.3 CN.multidot.H.sub.2 O (III)
(II) being obtained by oxidation with hydrogen peroxide and (III) by electrochemical oxidation of (II) in acetonitrile.
Hodgson et al., J. Am. Chem. Soc., 1990, 112, 6248-6254, restate that the possible use of bis(.mu.-oxo) dimanganese complexes as redox catalysts follows from the preliminary observations of Gref et al., Nouv. J. Chem., 1984, 8, 615-618, who electrochemically oxidized alcohols and ethers in the presence of bpy and phen complexes of manganese (see above), and from the studies by Ramaraj et al., Angew. Chem. Int. Ed. Engl., 1986, 25, 825-827, who showed that the bpy complex oxidized water in the presence of a chemical oxidizing agent such as the cerium(IV) ion.
In the above publication, D. J. Hodgson et al. restate or describe the preparation of complexes of the type of general formula (IV): EQU [(L)MnO.sub.2 Mn(L)].sup.2+ or 3+ (IV)
accessible from Mn(II) by oxidation with hydrogen peroxide in the presence of the ligand (n) of general formula (V) or (VI): ##STR1## in which: R.sub.1 =R.sub.2 =H; X=C; (Bispicen) or
R.sub.1 =CH.sub.3 ; R.sub.2 =H; X=C; or PA0 R.sub.1 =H; R.sub.2 =CH.sub.3 ; X=C; PA0 R.sub.1 =H; X=N; PA0 or else: ##STR2## in which: R.sub.1 =R.sub.2 =H; n=1 [TPA]; or PA0 R.sub.1 =CH.sub.3 ; R.sub.2 =H; n=2; or PA0 R.sub.1 =R.sub.2 =CH.sub.3 ; n=1. PA0 -(2-(2-pyridyl)ethyl)bis(2-pyridylmethyl)amine (L.sub.1), PA0 -(1-(2-pyridyl)ethyl)(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine (L.sub.2), PA0 -(6-methyl-2-pyridylmethyl)(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine (L.sub.3), PA0 -(6-methyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine (L.sub.4). PA0 Mn is manganese in the IV oxidation state, PA0 n and m independently have the value of integers from 2 to 8, PA0 X can represent a coordinating or bridging group such as H.sub.2 O, OH.sup.-, O.sub.2.sup.2-, HO.sub.2.sup.-, SH.sup.-, S.sup.2-, -SO-, NR.sub.2.sup.-, RCOO.sup.-, NR.sub.3, Cl.sup.-, N.sub.3.sup.-, SCN.sup.- or N.sup.3-, or a combination thereof, with R representing H, alkyl or aryl (optionally substituted), PA0 is a integer from 0 to 32, preferably 3 to 6, PA0 Y is a counterion whose type depends on the charge z of the complex; if z is positive, then Y represents an anion such as Cl.sup.-, Br.sup.-, I.sup.-, NO.sub.3.sup.-, ClO.sub.4.sup.-, NCS.sup.-, PF.sub.6.sup.-, RSO.sub.3.sup.-, RSO.sub.4.sup.-, CF.sub.3 SO.sub.3.sup.-, BPh.sub.4.sup.- 0 or OAc.sup.- ; if z is negative, then Y is a cation of an alkali metal or alkaline-earth metal or alternatively an (alkyl)ammonium cation, PA0 z is a negative or positive integer, PA0 q=z/charge of Y, PA0 L is a ligand which is an organic molecule containing a certain number of heteroatoms (N, P, O and S), some of which are coordinated to the manganese atoms. PA0 Mn represents manganese in a Ill or IV oxidation state, it being possible for the two Mn atoms of this cation to form an pair in the III-III or III-IV or IV-IV oxidation state, PA0 n has the value 2, 3 or 4, PA0 O representing oxygen, PA0 L represents a ligand of general formula (VIII): ##STR3## in which either R.sub.1 represents the radical: ##STR4## R.sub.2 then representing the radical: ##STR5## R.sub.3, R'.sub.3 and R".sub.3 each representing, independently of one another, a group chosen from hydrogen, C.sub.1 to C.sub.4 lower alkyl, C.sub.1 to C.sub.4 lower alkoxy or halogen, PA0 or R.sub.1 represents the radical: ##STR6## R.sub.2 and R'.sub.2 then being identical and representing a group chosen from hydrogen or C.sub.1 to C.sub.4 lower alkyl, PA0 R.sub.3 representing a group chosen from hydrogen, C.sub.1 to C.sub.4 lower alkyl, C.sub.1 to C.sub.4 lower alkoxy or halogen.
The electrochemical properties of these complexes were studied by cyclic voltametry in acetonitrile.
Hodgson et al., Inorg. Chem., 1990, 29, 2435-2441, have synthesized new ligands related to those above:
These ligands L.sub.1, L.sub.2, L.sub.3 and L.sub.4 lead to di-Mn complexes of the type restated above and catalyse the epoxidation of cyclohexene in the presence of iodosobenzene, which acts as the primary oxidizing agent.
The complex (L.sub.3) Mn(III)O.sub.2 Mn(IV)(L.sub.3) (ClO.sub.4).sub.3 was prepared by oxidation using hydrogen peroxide and then oxidized with NaOCl in acid medium to give the complex: EQU (L.sub.3)Mn(IV)O.sub.2 Mn(IV)(L.sub.3)(ClO.sub.4).sub.4
Patent Application EP 0,458,398 (Unilever NV and plc) relates to a bleaching medium comprising a peroxy bleaching agent and a manganese coordination complex (or a precursor of the latter) for use in washing and bleaching substrates, especially for textile whitening or washing dishes.
The latter coordination complex has the general formula (A): EQU [L.sub.n Mn.sub.m X.sub.p ].sup.z Y.sub.q (A)
in which:
Those which are preferred, from this family of complexes of formula (A), are of general formula (C): EQU [(L)Mn(IV)(.mu.-O).sub.3 Mn(IV)(L)].sup.z Y.sub.q (C)
in which L, Y, q and z are identical to those of the formula (A).
A preferred class of ligands L which correspond to the formula (C) are tridentate ligands which coordinate each manganese(IV) centre with 3 nitrogen N atoms.
Bis(pyrid-2-ylmethyl)amine appears among 19 of the latter (containing 3 nitrogen atoms).
The peroxy bleaching agents used comprise hydrogen peroxide (H.sub.2 O.sub.2), compounds which release or generate H.sub.2 O.sub.2, especially sodium perborate, and peroxy acids and their salts.
All the examples of this patent application (EP 0,458,398) relate to the bleaching, using these complexes, of stained cotton textile, at basic pH values between 10 and 11.
The problem of selectively delignifying lignocellulose materials without excessively depolymerizing the cellulose and without having industrial effluents which are environmentally undesirable remains current.
As restated by H. U. Suss, Bleaching, Vol. A4, 191-199, 1985, the bleaching of pulp in the paper industry by oxidizing agents exhibits disadvantages inherent in the properties of the oxidizing agent used or in the physicochemical conditions of the process employed.
Oxygen, for example, has little selectivity and also severely degrades the cellulose in NaOH medium and, to a lesser degree, in the presence of magnesium salts.
Chlorine is a relatively selective delignifying agent. Under the acidic conditions used, it causes oxidation but also electrophilic substitution of the aromatic rings of the lignin, producing dicarboxylic acids and chlorinated fragments of the lignin. The latter represent a potential danger to the environment.
Hydrogen peroxide is used essentially in basic medium. This basic medium leads to a certain depolymerization of the cellulose.